Intelligent transportation systems (ITS) encompass a broad range of wireless and wire line communications-based information and electronics technologies. Integrating ITS into the transportation system's infrastructure and in vehicles may improve safety on the roads, e.g., passenger and pedestrian safety, improve transportation productivity through the use of vehicle-to-vehicle and vehicle-to-roadside wireless communication technologies, and relieve congestion. For instance, investigations report that 17-37% of car accidents and 60% of roadway collisions could be prevented with development of the Intelligent Transportation Systems (ITS) technologies.
Due to the high mobility of vehicles, however, communications in vehicular networks need to satisfy strict delay requirements, which are usually less than a few hundreds of milliseconds. The United States Federal Communications Commission (US FCC) has approved 5.9 GHz spectrum for Dedicated Short Range Communications (DSRC) aiming to provide real-time, high speed links for vehicular networks. DSRC aims to support short duration wireless communications in rapid changing environments. Currently, the ASTM E2213-03 standard is being migrated to the IEEE 802.11p standard, and a new operation mode, named Wireless Access in Vehicular Environments (WAVE) mode, is added to IEEE 802.11.
ITS deployments require each vehicle to be equipped with an On Board Unit (OBU) performing wireless communications with the other vehicles or Road Side Units (RSUs). Briefly, equipment in the DSRC service comprises OBUs and RSUs. An OBU may be a transceiver that is installed in or on a vehicle, or a portable unit. An RSUs may be a transceiver that is mounted along a road or pedestrian passageway. An RSU may also be mounted on a vehicle or hand carried, but it may only operate when the vehicle or hand-carried unit is stationary. An RSU broadcasts data to OBUs or exchanges data with OBUs in its communications zone.
Certainly, the installation of OBUs on modern vehicles burdens the ever-increasing fuel consumption over the automotive electronic devices. Besides, the market has seen an increasing adoption of portable navigation devices (e.g., PDA) which may include OBU functions in the future. As significant gains in energy conservation are not likely to advent through refinement of the mature drivetrain (e.g., internal combustion engine) or battery technologies, there is a need for an energy-conservative communication protocol that improves the fuel/battery economy.
One direct way for DSRC to achieve fuel/energy conservation is to inherit the Power Saving (PS) mode from IEEE 802.11 standard. In IEEE 802.11 PS mode, the time axis on a station (in this case, OBU) is divided evenly into beacon intervals. Beacon interval in IEEE 802.11 refers to the amount of time between beacon transmissions. Before a station enters power save mode, the station uses the beacon interval to know when to wake up to receive the beacon. IEEE 802.11 PS mode allows an idle station to sleep a portion of each beacon interval. In IEEE 802.11 PS mode, an auxiliary timer synchronization mechanism is required to ensure the overlap of awake periods. Two power saving vehicles can communicate with each other only when their timers are synchronized. Since the topology of a vehicular network is frequently partitioned, making the timer synchronization costly or even infeasible, the IEEE 802.11 PS mode may not be practicable in vehicular networks.
Based on the IEEE 802.11 PS mode, a number of studies have explored Asynchronous Quorum-based Power Saving (AQPS) protocols. In an AQPS protocol, a station may stay awake or sleep during each beacon interval. Given an integer n, a quorum system defines a cycle pattern, which specifies the awake/sleep schedule during n continuous beacon intervals, for each station. Herein, n is referred to as the cycle length since cycle patterns repeat every n continuous beacon intervals. AQPS protocols ensure the asynchronous overlap of awake periods between stations; that is, during each cycle, an awake beacon interval of a station is guaranteed to overlap that of another station, and data communications can be successfully performed at these overlapped intervals, even if their boundaries are not aligned.
While AQPS protocols require no timer synchronization mechanism and may exhibit better feasibility for wide-scale, high-mobility vehicular networks, in most AQPS protocols the degree of power saving is limited by a theoretical bound. Given a cycle length n, a station is required to remain fully awake at least √{square root over (n)} beacon intervals per cycle to preserve the asynchronous overlap. The duty cycle of a station (i.e., portion of time a station must remain awake) can be no less than O(√{square root over (n)}/n)=O(1√{square root over (n)}). Note the neighbor discovery time increases proportionally to n due to the fact that the overlap may occur merely once every n beacon intervals. In presence of high mobility, the value of n should be set small to allow valid neighbor maintenance on each station. Under such a condition, the lower-bound of duty cycle can seriously restrict the effectiveness of an AQPS protocol.
It remains a challenge to efficiently assemble a symmetric quorum system while keeping the quorum size small. Existing symmetric quorum systems are constructed by using exhaustive searches or assuming n=k2+k+1, where k is a prime power. To efficiently construct an asymmetric quorum system (i.e., a-quorums and s-quorums) is even more challenging.